The astrological Nadir, more properly the Imum Coeli, is the lowest point of the ecliptic in a chart, the point where the ecliptic crosses the celestial meridian on its lower, subterranean branch, directly opposite the Midheaven. It is the cusp of the fourth house and one of the four angles.
The Latin imum coeli means "bottom of the sky," mirroring medium coeli, "middle of the sky," for its opposite point, both terms describing where the meridian, the great circle through the north and south horizon points and the zenith, meets the ecliptic. Because it lies at the meridian's lower culmination, always below the horizon at the chart's own moment, the Imum Coeli was described in some Hellenistic sources as the point farthest from the visible, rising part of the sky, and associated for that reason with the underworld, even though quadrant based house division, of the kind Ptolemy's Tetrabiblos and later systems use, treats it as the ordinary cusp of the fourth house rather than a purely symbolic point. The everyday English word "nadir," for the lowest point of anything, actually derives from a separate Arabic term for the point opposite the zenith and is only loosely applied here; the properly astrological term for this angle has always been Imum Coeli.